Collapse
When the North Tower collapsed on September 11, 2001, debris hit 7 WTC, causing heavy damage to the southern part.[5] The lower part of that southern part of the building was heavily affected: damage to the southeast corner, from the 8th to the 18th floor; a large vertical crack in the lower central part that extended at least ten stories; and other damage to already very high floors, such as the 18th.[5] The 7 WTC "World Trade Center (1973-2001)") was equipped with a fire sprinkler system, but the system had many vulnerable points, including requiring manual activation of the electric fire pumps, so it was not a completely automatic system. The sprinkler controls on each floor had just a single connection to activate the pumps, and the system required more power than the pumps provided to push the water. A loss of energy in the pumps and damage to the structure meant that the sprinklers did not work, in addition to the fact that the water pressure was low and insufficient to supply them.[28][29].
After the collapse of the North Tower, some firefighters entered 7 WTC to check the building. They attempted to extinguish any remaining fire, but low water pressure hampered their efforts.[30] A large fire burned in the afternoon on the 11th and 12th floors and the flames were visible from the east side of the building.[31][32] During the afternoon, flames were seen on the 6th to 10th, 19th to 22nd, and 29th and 30th.[5] At approximately 2:00 p.m. At around 3:30 p.m., firefighters noticed a bulge in the southwest corner of 7 WTC, between the 10th and 13th floors, which was a sign that the building was unstable and could collapse. m. and, given that 7 WTC would possibly collapse, Daniel Nigro, head of the FDNY, decided to suspend rescue operations, the removal of remains and the search through the rubble in the vicinity of the building, evacuating the area to guarantee the safety of personnel.[35][33] At 5:20 p.m. m., 7 WTC collapsed, but had already been evacuated, so there were no deaths.
In May 2002, the Federal Emergency Management Agency (FEMA) released a report on the collapse, based on a preliminary investigation conducted jointly with the Structural Engineering Institute of the American Society of Civil Engineers, under the direction of W. Gene Corley, P.E. FEMA made preliminary conclusions indicating that the collapse was not only caused by the damage caused by the collapse of the Twin Towers "World Trade Center (1973-2001)," but also by the fire started in different places by falling debris from the Twin Towers, and which continued burning due to the lack of water in the sprinklers and manual fire-fighting measures. The structural elements were exposed to high temperatures for a long enough time to see their load capacity reduced to the point of causing collapse.[8] The report did not draw final conclusions about the cause of the collapse but did point out several points that required more detailed investigation. FEMA concluded:
In response to FEMA's concern, the Department of Commerce's National Institute of Standards and Technology (NIST) was tasked with conducting an investigation into the structural failure and collapse of the Twin Towers "World Trade Center (1973-2001)" and 7 WTC. The investigation, led by Dr. S. Shyam Sunder, lasted 3 years and cost $16 million.[36] It was not an internal expert investigation but relied on the advice of several external private institutions, such as the "Structural Engineering Institute of the American Society of Civil Engineers" (SEI/ASCE), the "Society of Fire Protection Engineers" (SPFE), the "National Fire Protection Association" (NFPA), the "North American Institute of Steel Construction" (AISC), the "City Council on Tall Buildings and Urban Habitat" (CTBUH) and the "Structural Engineers Association of New York" (SEAoNY).[37].
Some existing photos and videos show the damage caused to the southern part of 7 WTC on 9/11. For example, from a helicopter, ABC News obtained images of the south facade of 7 WTC that show a deep crack, extending approximately 10 floors. NIST released a video and a sequence of photos of 7 WTC before its collapse that seemed to indicate the existence of greater structural damage than originally assumed by FEMA, and that it was directly caused by falling debris. Specifically, NIST's interim report on 7 WTC featured photographs of the building's southwest façade that showed significant damage. The report highlights a 10-story crack in the center of the south façade, which extends inward approximately a quarter of its width. cracks and a tilt of the east facade of the building immediately after the collapse, which began on the attic floors.[5] In the video of the collapse, taken from the north side by CBS News and other media outlets, the first visible sign of the collapse is the movement in attic 8.2, seconds before the north facade began to collapse, and which lasted for at least another 7 seconds.[5][40].
A progress report on the investigation was published in June 2004, summarizing the hypothesis NIST was working on.[41][5] The hypothesis, reiterated in June 2007 in an update to the report, was an initial failure of a critical support located under the 13th floor, caused by fire and/or debris from the World Trade Center (1973-2001) Twin Towers. The collapse progressed vertically to the east attic facilities floor. The interior structure was unable to support the redistribution of the load, causing the horizontal progression of the failure to the ground floors, especially from the 5th to the 7th. This resulted in "a disproportionate collapse of the entire structure."[42]
NIST anticipated publishing a draft of the 7 WTC investigation in 2008.[43] NIST used ANSYS to model the events that led to the onset of the crash and LS-DYNA models to simulate the global reaction to the initiated events.[44] The 7 WTC investigation has been delayed for numerous reasons, one of which was that some of the NIST staff who had been working on the 7 WTC investigation were reassigned full-time, from June 2004 to September 2005, to work on the Twin Towers collapse investigation.[45] In June 2007, NIST explained: "We are moving forward as quickly as possible, while rigorously testing and evaluating a wide variety of scenarios to reach the most definitive conclusion possible. The investigation of 7 WTC is, in some respects, as challenging, if not greater, than that of the towers. However, the current study benefits greatly from the significant technological advances made and the lessons learned from our work on the towers.
Some conspiracy theorists believe that the 9/11 collapses, including 7 WTC, were the result of controlled demolitions.[46][47] NIST "has found no evidence of an explosion or controlled demolition events." In their final report on 7 WTC they wanted to "determine the magnitude of the different hypothetical explosion scenarios that could have led to the structural failure of one or more critical elements."[45].
When 7 WTC collapsed, its debris caused substantial damage and ruin to the Borough of Manhattan Community College's Fiterman Hall building, located at 30 West Broadway. The building was rebuilt in August 2007.[48] Another adjacent building, the Verizon Building, an art deco tower built in 1926, suffered extensive damage to its east façade in the collapse of 7 WTC, although it was restored at a cost of $1.4 billion.[49]